Phase shift masks include attenuator, rim shift, and alternating types. While attenuator masks provide a layer of material on the quartz substrate, rim shift and alternating phase shift masks can involve etching the quartz substrate itself to provide differential phase transmission. One of the key problems in fabricating any of these masks is the difficulty of repairing defects formed during the mask manufacturing process. A defect may arise because of the presence of a minute particle on the quartz substrate, in the chromium layer on the quartz substrate, or in the photosensitive material used to pattern the mask. The problem is particularly severe where the quartz substrate is etched because heretofore there has been no repair or rework method available for defects in the quartz. Thus, defects formed in the quartz after the etch step have been regarded as unrepairable, and masks with quartz defects have been scrapped. Because defects are pervasive in each of the steps of mask fabrication, the inability to repair quartz defects has sharply limited the ability to fabricate alternating phase shift masks and rim shift masks that require quartz etching.
In fabricating these alternating phase shift masks, a blanket layer of chromium on the quartz substrate is first photolithographically patterned to provide desired shapes of chrome, such as boxes and lines, and spaces there between. Then the quartz is etched in locations defined by the spaces between chromium patterns.
The etch is carefully controlled to create trenches having a specified depth so light passing through the region with a trench passes through a quartz substrate thinned compared to light passing through a neighboring region that has the full thickness of quartz, the difference in thicknesses providing a 180.degree. phase difference in the light transmitted by each. Because of the interference of the out of phase light there is a significant intensification of contrast between adjacent regions when light is shined through the mask, and this intensification of contrast provides the ability to resolve significantly smaller structures than can be achieved without phase shifting. Typically, the etch depth is controlled to provide a relative shift between the two neighboring regions of half of the wavelength of the light used with the mask to expose semiconductor wafers, thereby providing the 180.degree. phase difference.
Defects in the etching of the quartz are analogous to the opaque and clear defects of standard masks, described in commonly assigned co-pending patent application Ser. No. 08/786,061, (the '061 application). Thus, analogous to opaque defects are regions where quartz should have been etched but was not etched. In these regions of the mask there remain unwanted columns of full thickness quartz. Analogous to clear defects, are regions where quartz was etched but should not have been etched. In this case pits or divots have been etched in the quartz in regions that should have remained full thickness. The two types of defects can be in isolated regions or they can be adjacent or attached to a wanted region of full thickness quartz or to a region protected by chromium.
A solution is needed that provides for rapidly, accurately, reproducibly and reliably repairing defects in the quartz of phase shift masks, and this solution is provided by the following invention.